Take a look at @EduardoRFS 's reason-mobile project.
And I wrote a post recently about Android and universal macOS apps (Cross-compiling OCaml with GitHub Actions - #3 by jbeckford) which you may want to skim read. But from what I gather you should be looking at reason-mobile because: I don’t have iOS bundled yet; reason-mobile is a complete solution today; you are already heavy into the Javascript side of OCaml.
I’m fairly sure you will be waiting a while for multicore support for Android and iOS, if that is what you meant by “multiplatform”. Multicore on other supported platforms will be ahead in the queue.
I’d be curious to know if you really want to use multiple cores for OCaml in a mobile app. IMHO: Since there are usually only a couple high-performance cores on an everyday phone, and since overuse / incorrect use of mutexes is very noticeable (bad) to a mobile user, I want concurrency but not parallelism in mobile apps. Concurrency in OCaml 4.x is good, and concurrency in OCaml 5.x with effects handlers should be great. I want OCaml 5.x multicore (parallelism) really only for running OCaml faster+cheaper on backend Linux servers